Field of the Invention
The present invention relates to a multilayer ceramic capacitor whose ratio of the concentration of the specified donor element and that of the specified acceptor element in the ceramic grain constituting the dielectric layer varies depending on the site of the ceramic grain.
Description of the Related Art
In recent years, there is a high demand for smaller electronic components to support higher density electronic circuits used in mobile phones, tablet terminals, and other digital electronic devices, and accordingly, multilayer ceramic capacitors (MLCC) that constitute these circuits are rapidly becoming smaller in size and larger in capacitance.
The capacitance of a multilayer ceramic capacitor is proportional to the dielectric constant of the constituent material of the dielectric layers constituting the capacitor and also to the number of the dielectric layers, and inversely proportional to the thickness of one dielectric layer. For this reason, answering the demand for smaller multilayer ceramic capacitors requires that the dielectric constant of their material be increased, while the thickness of the dielectric layer be reduced to increase the number of layers.
However, reducing the thickness of the dielectric layer increases the voltage applied per unit thickness, which in turn shortens the service life of the dielectric layer and consequently reduces the reliability of the multilayer ceramic capacitor. Accordingly, dielectric compositions to which such donor elements as Mo and W are added to improve the service life, are proposed.
In addition, the distribution of abundance ratios of the aforementioned donor elements and other additive elements in the ceramic grains constituting the dielectric layer affects the performance of the multilayer ceramic capacitor. About this point, Patent Literature 1, for example, describes a dielectric porcelain offering improved dielectric breakdown voltage, wherein the dielectric porcelain is such that additive elements such as Mn, V, Cr, Co, Ni, Fe, Nb, Mo, Ta, and W (these components are believed to improve reduction resistance) are distributed roughly uniformly over the entire range from the grain boundary to the center of the crystal grain.
Patent Literature 2 proposes a multilayer ceramic capacitor whose service life will not be shortened due to dielectric breakdown, etc., even if the number of dielectric layers is increased or the thickness of the dielectric layer is reduced, and which also permits size reduction and capacitance increase, wherein the constitution of the multilayer ceramic capacitor is such that its ceramic grain is constituted by a crystalline core and a shell that encloses the core, that additive elements such as Mn, V, Cr, Mo, Fe, Ni, Cu, and Co are added to the core, and that the concentration of these additive elements increases from the center of the core toward the shell.
Also, Patent Literature 3 describes a barium titanate ceramic grain as a dielectric ceramic that will give a multilayer ceramic capacitor offering good capacitance-temperature characteristics and excellent service life characteristics, wherein such grain is characterized in that it has a core and a shell and contains rare earth elements R and M (here, M represents at least one type selected from the group that consists of Mg, Mn, Ni, Co, Fe, Cr, Cu, Al, Mo, W, and V) as secondary constituents, where the total concentration of R and M slopes from the grain boundary to the core and becomes minimum in a part and maximum in other part.